Semiconductor memory devices are typically implemented using semiconductors such as silicon (Si), germanium (Ge), gallium arsenide (GaAs), and indium phosphide (InP). In general, semiconductor memory devices are classified into volatile memory devices and nonvolatile memory devices.
Volatile memory devices lose their stored data when their power supplies are interrupted. Volatile memory devices include static RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), and the like. Nonvolatile memory devices retain their stored data even when their power supplies are interrupted. Nonvolatile memory devices include read only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable and programmable ROM (EEPROM), flash memory, phase-change RAM (PRAM), magnetic RAM (MRAM), resistive RAM (RRAM), ferroelectric RAM (FRAM), and the like.
Due to high access speeds, DRAMs have been widely adopted in the marketplace, and can be used, for example, as a working memory, a buffer memory, and a main memory of a computing system. At a fundamental level, a memory cell of a DRAM includes a transistor and a capacitor. A DRAM stores data by charging or discharging electric charges to or from the capacitor. But the charge in the capacitor leaks out over time. Accordingly, the DRAM periodically performs a refresh operation to retain the data stored in the DRAM. When the refresh operation is not performed sufficiently, the stored data may be lost.